Around 1000 BC, early fortifications were erected on the Festung Ehrenbreitstein hill on the opposite side of the Moselle. In 55 BC, Roman troops commanded by Julius Caesar reached the Rhine and built a bridge between Koblenz and Andernach. About 9 BC, the "Castellum apud Confluentes", was one of the military posts established by Drusus.

Remains of a large bridge built in 49 AD by the Romans are still visible. The Romans built two castles as protection for the bridge, one in 9 AD and another in the 2nd century, the latter being destroyed by the Franks in 259. North of Koblenz was a temple of Mercury and Rosmerta (a Gallo-Roman deity), which remained in use up to the 5th century.

With the fall of the Western Roman Empire, the city was conquered by the Franks and became a royal seat. After the division of Charlemagne's empire, it was included in the lands of his son Louis the Pious (814). In 837, it was assigned to Charles the Bald, and a few years later it was here that Carolingian heirs discussed what was to become the Treaty of Verdun (843), by which the city became part of Lotharingia under Lothair I. In 860 and 922, Koblenz was the scene of ecclesiastical synods. At the first synod, held in the Liebfrauenkirche, the reconciliation of Louis the German with his half-brother Charles the Bald took place. The city was sacked and destroyed by the Normans in 882. In 925, it became part of the eastern German Kingdom, later the Holy Roman Empire.

In 1018, the city was given by the emperor Henry II to the archbishop and prince elector of Trier after receiving a charter. It remained in the possession of his successors until the end of the 18th century, having been their main residence since the 17th century. Emperor Conrad II was elected here in 1138. In 1198, the battle between Philip of Swabia and Otto IV took place nearby. In 1216, prince-bishop Theoderich von Wied donated part of the lands of the basilica and the hospital to the Teutonic Knights, which later became the Deutsches Eck.

In 1249–1254, Koblenz was given new walls by Archbishop Arnold II of Isenburg; and it was partly to overawe the turbulent citizens that successive archbishops built and strengthened the fortress of Ehrenbreitstein that still dominates the city.

The city was a member of the league of the Rhenish cities which rose in the 13th century. The Teutonic Knights founded the Bailiwick of Koblenz in or around 1231. Koblenz attained great prosperity and it continued to advance until the disaster of the Thirty Years' War brought about a rapid decline. After Philip Christopher, elector of Trier, surrendered Ehrenbreitstein to the French, the city received an imperial garrison in 1632. However, this force was soon expelled by the Swedes, who in their turn handed the city over again to the French. Imperial forces finally succeeded in retaking it by storm in 1636.

In 1688, Koblenz was besieged by the French under Marshal de Boufflers, but they only succeeded in bombing the Old City (Altstadt) into ruins, destroying among other buildings the Old Merchants' Hall (Kaufhaus), which was restored in its present form in 1725. The city was the residence of the archbishop-electors of Trier from 1690 to 1801.

After World War I, France occupied the area once again. In defiance of the French, the German populace of the city has insisted on using the more German spelling of Koblenz since 1926. During World War II it was the location of the command of German Army Group B and like many other German cities, it was heavily bombed and rebuilt afterwards. Between 1947 and 1950, it served as the seat of government of Rhineland-Palatinate.

Its defensive works are extensive, and consist of strong forts crowning the hills encircling the city to the west, and the citadel of Ehrenbreitstein on the opposite bank of the Rhine. The old city was triangular in shape, two sides being bounded by the Rhine and Mosel and the third by a line of fortifications. The latter were razed in 1890, and the city was permitted to expand in this direction. The Koblenz Hauptbahnhof (central station) was built on a spacious site outside the former walls at the junction of the Cologne-Mainz railway and the strategic Metz-Berlin line. In April 2011 Koblenz-Stadtmitte station was opened in the inner city to coincide with the opening of the Federal Garden Show 2011. The Rhine is crossed by the Pfaffendorf Bridge, originally the location of a rail bridge, but now a road bridge and, a mile south of city, by the Horchheim Railway Bridge, consisting of two wide and lofty spans carrying the Lahn Valley Railway, part of the Berlin railway referred to above. The Moselle is spanned by a Gothic freestone bridge of 14 arches, erected in 1344, two modern road bridges and also by two railway bridges.

Since 1890, the city has consisted of the Altstadt (old city) and the Neustadt (new city) or Klemenstadt. Of these, the Altstadt is closely built and has only a few fine streets and squares, while the Neustadt possesses numerous broad streets and a handsome frontage along the Rhine.

In the more ancient part of Koblenz stand several buildings which have a historical interest. Prominent among these, near the point of confluence of the rivers, is the Basilica of St. Castor or Kastorkirche, dedicated to Castor of Karden, with four towers. The church was originally founded in 836 by Louis the Pious, but the present Romanesque building was completed in 1208, the Gothic vaulted roof dating from 1498. In front of the church of Saint Castor stands a fountain, erected by the French in 1812, with an inscription to commemorate Napoleon's invasion of Russia. Not long after, Russian troops occupied Koblenz; and St. Priest, their commander, added in irony these words: "Vu et approuvé par nous, Commandant russe de la Ville de Coblence: Janvier 1er, 1814."

In this quarter of the city, too, is the Liebfrauenkirche, a fine church (nave 1250, choir 1404–1431) with lofty late Romanesque towers; the castle of the electors of Trier, erected in 1280, which now contains the municipal picture gallery; and the family house of the Metternichs, where Prince Metternich, the Austrian statesman, was born in 1773. Also notable is the church of St. Florian, with a two towers façade from c. 1110.

The former Jesuit College is a Baroque edifice by J.C. Sebastiani (1694–1698) serves as the current City Hall.

Near Koblenz is the Lahneck Castle near Lahnstein, open to visitors from 1 April to 31 October.

In the modern part of the city lies the palace (Residenzschloss), with one front looking towards the Rhine, the other into the Neustadt. It was built in 1778–1786 by Clement Wenceslaus, the last elector of Trier, under design by the French architect P.M. d'Ixnard. In 1833, the palace was used as a barracks, and became the final depot for the optical telecommunications system that originated in Potsdam. Today, the elector's former palace is a museum; among other curiosities, it contains some fine Gobelin tapestries. From it some pretty gardens and promenades (Kaiserin Augusta Anlagen) stretch along the bank of the Rhine, and in them is a memorial to the poet Max von Schenkendorf. A fine statue to the empress Augusta, whose favourite residence was Coblenz, stands in the Luisenplatz. But of all public memorials the most striking is the colossal equestrian statue of the emperor William I of Germany, erected by the Rhine provinces in 1897, standing on a lofty and massive pedestal, at the point where the Rhine and Mosel meet.

In 1897, a monument to German EmperorWilliam I of Germany, mounted on a 14-metre-high horse, was inaugurated there by his grandson William II. The architect was Bruno Schmitz, who was responsible for a number of nationalistic German monuments and memorials. The German Corner is since associated with this monument, the (re) foundation of the German Empire and the German refusal of any French claims to the area, as described in the song "Die Wacht am Rhein" together with the "Wacht am Rhein" called "Niederwalddenkmal" some 30 kilometres (19 miles) upstream.

During World War II, the statue was destroyed by US artillery. The French occupation administration intended the complete destruction of the monument and wanted to replace it with a new one.

In 1953, Bundespräsident Theodor Heuss re-dedicated the monument to German unity, adding the signs of the remaining western federal states as well as the ones of the lost areas in the East. A Flag of Germany waved there since. The Saarland was added four years later after the population had voted to join Germany.

In the 1980s, a movie of the monument was often shown on late night TV when the National Anthem was played to mark the end of the day, a practise which was discontinued when nonstop broadcasting became common. On 3 October 1990, the very day the former GDR states joined, their signs were added to the monument.

As German unity was considered complete and the areas under Polish administration were ceded to Poland, the monument lost its official active purpose, now only reminding of history. In 1993, the flag was replaced by a copy of the statue, donated by a local couple. The day chosen for the reinstatement of the statue, however, caused controversy as it coincided with Sedantag (Sedan Day) (2 September 1870) a day of celebration remembering Germany's victory over France in the Battle of Sedan.[2] The event was widely celebrated from the 1870s until the start of the 20th century.

Koblenz is a principal seat of the Mosel and Rhenish wine trade, and also does a large business in the export of mineral waters. Its manufactures include automotive parts (braking systems – TRW Automotive, gas springs and hydraulic vibration dampers – Stabilus), aluminium coils (Aleris Aluminum), pianos, paper, cardboard, machinery, boats, and barges. Since the 17th century, it has been home to the Königsbacher brewery (the Old Brewery in Koblenz's city centre, and now a plant in Koblenz-Stolzenfels). It is an important transit centre for the Rhine railways and for the Rhine navigation.